make thread killlock async-signal-safe for pthread_kill

make thread killlock async-signal-safe for pthread_kill

pthread_kill is required to be AS-safe. that requirement can't be met
if the target thread's killlock can be taken in contexts where
application-installed signal handlers can run.

block signals around use of this lock in all pthread_* functions which
target a tid, and reorder blocking/unblocking of signals in
pthread_exit so that they're blocked whenever the killlock is held.